A light emitting apparatus which radiates light with color different from the light of a light emitting element has been developed. The light emitting apparatus converts a part of the light of the light emitting element into a different wavelength by a phosphor, and mixes the light with converted wavelength and the light with non-converted wavelength of the light emitting element (for example, Japanese Laid-Open Publication Kokai No. 2002-198573). For example, a while LED light emitting apparatus, which has a blue light emitting diode (hereinafter, occasionally referred to as an “LED”) and a phosphor member coating thereon, has been in actual use. The blue light emitting diode employs an InGaN group material is used as a light emitting element. The phosphor member is composed of a transparent material, such as an epoxy resin, containing an Yttrium aluminum garnet (hereinafter, occasionally referred to as “YAG”) group phosphor represented by the formula (Y,Gd)3(Al,Ga)5O12. The luminescent color of the white LED light emitting apparatus is obtained based on the additive color mixture principle. After the blue light emitted from the LED enters into the phosphor member, it is repeatedly absorbed and scattered in its layer, and then is outwardly radiated. On the other hand, the blue light absorbed by the phosphor serves as an excitation source, and exhibits yellow fluorescence. Human eyes recognize the mixture of the yellow light from the phosphor and the blue light from the LED as white.
The LED light emitting apparatus using such an LED is small, highly effective in terms of electric power consumption, and emits vivid color light. Since an LED is a semiconductor element, it is not prone to burn out. In addition, it has features, such as excellent initial drive characteristics, resistance to vibration or ON/OFF repeats. Since LEDs have these excellent characteristics, LED light emitting apparatuses are used as various kinds of light sources.
However, since conventional white light emitting apparatuses employ resin much, there is a problem that the resin deteriorates in the case where a light emitting element with high power or light with short wavelength. In the case where a cured film which employs an inorganic group binder, in particular silica gel (SiO2), is used, there is a problem that the film is colored and deteriorates, and then darkens in exposure to high power or ultraviolet rays. Although the reason for this is not clear, it is assumed that an organic group included in silica sol remains therein after cured and thus is reduced by excitation of high-power light.
In order to improve light-outgoing efficiency of light emitting apparatus, improvement of the transmittance of luminescent film is a candidate. The transmittance of luminescent film depends on the transmittance of binder which binds a phosphor in the luminescent film. In the case where gel obtained by curing the sol is used as the binder, as the gel becomes closer to the polycrystal state by proceeding of sol-gel reaction as shown in FIG. 1, it is generally considered that the transmittance of the luminescent film increases as shown A in the Figure.
However, it is necessary to perform the reaction at high temperature to bring the gel close to the polycrystal state. This requires more time and energy. In addition, there is a problem that high temperature affects the semiconductor light emitting element or phosphor. For example, lead wires bonded to the LED chip can be damaged by the heat, or the phosphor deteriorates. Accordingly, in terms of reaction temperature, it is difficult to vitrify the sol to obtain inorganic polycrystalline by proceeding of the sol-gel reaction for improvement of light-outgoing efficiency.
Even if it is vitrified to obtain inorganic polycrystalline by proceeding of the sol-gel reaction, various problems arise in the interface between the luminescent film and the light emitting element. The problems arise, for example, total reflection occurs in the vitrified interface, thus, the light-outgoing efficiency and the extraction of reduces, or a space layer is formed in the interface of the light emitting element or the phosphor by a cause of the cure, thus, the space layer obstructs outgoing of the light.
Additionally, in the construction where the light emitting element such as LED excites a luminescent layer, there is also a problem that the luminescent layer deteriorates in exposure to high energy of excitation light from the LED. Since the light emitting layer that deteriorates becomes colored and turns to blackish, its original transparent characteristics deteriorates, thus, the light-outgoing efficiency reduces. Although this reason of the color deterioration of the darkening is not clear, the reason is assumed that the silica employed in the binder of the light emitting layer causes it.
Even if typical resins are used as a molding member which molds the phosphor in the light emitting layer, the resins remarkably deteriorate in exposure to high-power light. Accordingly, it is difficult to use resins as the molding member. For this reason, transparent binders, such as silica (SiO2), are used. Silica in a sol state has excellent binding characteristics and transparent characteristics, and provides excellent light-outgoing efficiency. In addition, it is inexpensive in industrially use. Accordingly, it is useful.
However, when exposed to high-power light from LED for a long time, a silica binder layer is colored and deteriorates. Particularly in a high-power light emitting apparatus, the silica binder layer deteriorates by the light with high density and the heat, and thus is colored black or dark brown. As a result of the research by the inventors of the present invention, this reason is considered that SiOx (x<2), which should be SiO2 as silica, is produced by oxygen omission. Under heat-curing temperature of 250° C., the silica binder is in a silica gel state where hydroxyl group and organic group partly remain in the SiO2 skeleton. When light with high density is incident from LED in a silica gel state, oxygen omission occurs, thus, SiO2 produces SiOx (x<2). As mentioned above, since Si is prone to be oxidized and reduced, that reason is considered that oxygen omission which occurs in the silica gel causes color deterioration. When color deterioration occurs, the problem that the light output from the light emitting element reduces arises.
Further, recently, a light emitting apparatus which uses a high-power light emitting element has been developed, it tends to accelerate the deterioration of resin by the light from the light emitting element. Moreover, efforts are moving ahead to develop light emitting elements with short wavelength such as the wavelength range from blue to short wavelength in the visible light region, in addition, with the ultraviolet region. On the other hand, a coating film which can endure ultraviolet rays or the like for a long time is not known. If typical resins are employed, the resins remarkably deteriorate in exposure to high-power light. Accordingly, it is difficult to use these resins as the coating film.
The present invention is aimed at solving the problems. It is a main object of the present invention to provide a luminescent film, a light emitting apparatus, a method for producing a luminescent film, and a method for producing a light emitting apparatus which improve light-outgoing efficiency and has excellent reliability, in addition, to provide a light emitting apparatus having a coating film which is less prone to deteriorate due to the light such as ultraviolet rays from a light emitting element.